Order Book
A live list of the prices and quantities people are willing to trade at, sorted from the best bid to the best offer.
Bids are resting orders to purchase at or below a stated price; asks are resting orders to sell at or above one. A matching engine pairs them, normally by price first and then by time of arrival, so an order that improves the price jumps the queue. An order that takes the best available price and moves on to the next price level is said to walk the book, which is where price impact comes from. Unlike regulated equity markets with a consolidated tape and a single national best quote, digital-asset books are fragmented across many venues, so there is no one official price at any instant.
In practice
A depth chart on an exchange draws the cumulative size of resting bids to the left of the current price and resting asks to the right.
The common misunderstanding
A large resting order is not a commitment; it can be canceled in the instant before it would trade, so a deep-looking book can thin out immediately.